In addition to a detailed cellular analysis of the relation of oscillatory after-potentials to arrhythmias induced by digitalis, and of the relation of after-contractions to the positive inotropic actions of the drug, we shall, in this final year, continue the developemnt of the sucrose-gap model of parasystole in canine Purkinje tissue. Specifically, we will assess the action of digitalis and other drugs on the patterns of arrhythmia generated by the model as a function of driving frequency. In addition, a closely related model of reentry ("reflection") can be produced in the same preparation. The relationship between parasystole and reentry can thus be studied in the same isolated tissue preparation. A comparison of frequency-dependent patterns of arrhythmia, and of the action of drugs on the two mechanisms, can be readily assessed.